The present invention relates generally to semiconductor integrated circuits. More particularly, it pertains to a method and structure for an improved floating gate memory cell.
Modem integrated circuit technology relies on transistors and memory cells to formulate vast arrays of functional circuits. The complexity of these circuits requires the use of an ever increasing number of linked transistors and memory cells. As the number of transistors and memory cells required increases, the surface area that can be dedicated to a single component dwindles. Today, also, high reliability is needed in each component to reduce the amount of redundancy needed to guarantee proper operation. Additionally, it is desired to have uniformity among memory cells to facilitate uniform erasure and avoid charge leakage for such memory devices. Thus, it is desirable to construct integrated circuit components which have higher reliability and greater uniformity that can accommodate higher density arrangement on the surface of the silicon chip.
Non volatile floating gate tunneling oxide (FLOTOX) devices, i.e. FLOTOX transistors, offer the prospect of very high density memory cell structures. Flash memories are one form of FLOTOX devices and electronically erasable and programmable read only memories (EEPROMs) are another. Due to their high density nature, memories formed with FLOTOX transistors have the potential of replacing hard storage disk drives in computer systems. The advantages to this substitution would be in replacing a complex and delicate mechanical system with a rugged and easily portable small solid-state non-volatile memory system. There is also the possibility that given more speed of operation, particularly in the erase operation, that FLOTOX transistors might be used to replace dynamic random access memories (DRAMs). Thus, FLOTOX transistors might eventually have the ability to fill all memory needs in future computer systems.
In operation, FLOTOX transistors can be electronically programmed, erased, and reprogrammed. In FLOTOX transistors a floating gate is electrically isolated and any charge stored on the floating gate is trapped. Storing sufficient charge on the floating gate will make it more difficult to form an inversion channel between the source and drain of the FLOTOX transistor. Thus, the presence or absence of charge on the floating gate represents two distinct data states.
Typically, FLOTOX transistors are selectively programmed, or xe2x80x9cwritten to,xe2x80x9d by hot electron injection which places a charge on a floating gate during a write. The FLOTOX transistors are selectively erased by Fowler-Nordheim tunneling which removes the charge from the floating gate. During a write, a high programming voltage is placed on a control gate. This forces an inversion region to form in the p-type substrate. The drain voltage is increased to approximately half the control gate voltage while the source is grounded, increasing the voltage drop between the drain and source. In the presence of the inversion region, the current between the drain and source increases. The resulting high electron flow from source to drain increases the kinetic energy of the electrons. This causes the electrons to gain enough energy to overcome the outside barrier and collect on the floating gate.
After the write is completed, the negative charge on the floating gate raises the transistor""s threshold voltage (VT) above the wordline logic 1 voltage. When a written transistor""s wordline is brought to a logic 1 during a read, the transistors will not turn on. Sense amplifiers detect and amplify the transistor current, and output a logic 0 for a written transistor.
The floating gate can be unprogrammed, or xe2x80x9cerased,xe2x80x9d by grounding the control gate and raising the source voltage to a sufficiently high positive voltage to transfer electrons out of the floating gate to the source terminal of the transistor by tunneling through the insulating gate oxide. After the erase is completed, the lack of charge on the floating gate lowers the cell""s VT below the wordline logic 1 voltage. Thus when an erased cell""s wordline is brought to a logic 1 during a read, the transistor will turn on and conduct more current than a written cell. Some flash devices use Fowler-Nordheim tunneling for write as well as erase.
One of the present hurdles in reducing the size of the FLOTOX transistor is related to the creation of the floating polysilicon gate and the shallow trench isolation region between adjacent FLOTOX transistors. Typically, the shallow trench isolation and the floating polysilicon gate are defined using two different lithography masks. Because of potential errors in the alignment of these two layers, the cell design must include extra area for the overlap of the floating polysilicon gate with the shallow trench isolation. In addition, the process steps required to form the shallow trench isolation, along with the processing steps between the formation of the shallow trench isolation region and the growth of the tunnel oxide/deposition of the floating polysilicon gate, create the potential for the thinning of the tunnel oxide.
Thinning of the tunnel oxide can lead to at least two performance problems. A first problem is erase uniformity. Flash memory is not erased on a cell by cell basis, but rather on a block by block basis. Because a large number of cells are erased at the same time, it is important that all of the cells within each block erase at close to the same rate. Any variation in field edge thinning will increase the cell to cell erase distribution. A second problem is data retention. The thinning of the tunnel oxide, at the field edges, can create localized tunneling at the these regions. Because of the higher fields and density of charge during erase, in these regions, long term data retention can become a problem.
Another problem arises when the device size is reduced, the gate coupling ratio (GCR) decreases. The surface area of the two gates is reduced when the overall size of the device is reduced. The GCR is a factor of the surface area of the two gates and is thereby decreased as the surface area of the gates diminishes. In order to overcome this loss in GCR, the periphery transistors must operate at higher voltage levels. If the loss of gate coupling is large enough, this may require the periphery transistors to operatate near their breakdown levels.
One method used to recover surface area lost as the overall device size is reduced is described in xe2x80x9cA Novel High-Density 5F2 NAND STI Cell Technology Suitable for 256 Mbit and 1 Gbit Flash Memoriesxe2x80x9d, K. Shimizu, K. Narita, H. Watanabe, E. Kamiya, T. Yaegashi, S. Aritome and T. Watanabe, 1997 IEDM 271-274. The method used was to 1) deposit a second polysilicon layer over the first polysilicon film, 2) a thin layer of silicon-nitride is then deposited over the second polysilicon layer, 3) a lithography level is then used to pattern the silicon nitride film only, 4) a second layer of silicon nitride is then deposited over the patterned silicon nitride film, 5) the second layer of silicon nitride next receives a blanket directional etch to form 2nd layer silicon nitride spacers on the patterned edges of the 1st layer of silicon nitride, 6) the final silicon nitride structure is than used as an etch mask to etch/remove the second polysilicon film over the shallow trench isolation, and 7) the silicon nitride mask is removed. The final outcome of all of the above processing is the addition of an extension over a certain percent of the shallow trench isolation. This process allows for very small gaps between the floating polysilicon gate, cell to cell. This leads to an increase in surface area between the floating gate and the control gate, with no increase in surface area between floating gate and the silicon substrate. The net result is increased coupling of the control gate to the floating gate. The problem with this approach is that the process becomes very complex. The process requires two additional silicon-nitride depositions, one additional lithography step, two additional silicon-nitride etch steps, one additional polysilicon etch step and finally the removal of the silicon-nitride mask.
Thus, what is needed is a method and apparatus for creating integrated circuit components which have higher reliability and greater uniformity and that can accommodate a higher density arrangement on the surface of the silicon chip without a loss in gate coupling ratio and with a simplified fabrication process.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, it is desirable to develop a method and structure for improved FLOTOX transistors.
The above mentioned problems with non volatile FLOTOX transistors and other problems are addressed by the present invention and will be understood by reading and studying the following specification. A method and structure for an improved floating gate memory cell are provided. The present invention capitalizes on a single step process for defining a shallow trench isolation (STI) region and floating gate that aligns the edges of each with the other. This alignment allows for a significant reduction in the size of each memory cell that correspondingly increases the density of memory cells in a particular area of a memory system. The alignment also facilitates an improved edge profile of the floating gate polysilicon that correspondingly increases data retention due to the elimination of localized tunneling at the field isolation edge. Additionally, the alignment provides for greater erase uniformity by reducing variations due to tunnel oxide thinning at the edge of the field isolation uniformity.
The present invention also employs techniques to increase the area of the top side of the floating gate. The area increase is needed to compensate for the loss in area due to the reduced size of the memory cell that correspondingly reduces the gate coupling ratio (GCR) between the floating gate and the control gate. The techniques include using hemispherical grain (HSG) polysilicon or wings on the top surface of the floating gate. Either HSG polysilicon or wings will increase the surface area of the top side of the floating gate. The increased area maintains or improves the GCR between the floating gate and the control gate of the memory cell, thus, allowing the memory cell to function at reasonable signal levels.
In particular, one embodiment of the present invention includes a non volatile memory cell. The non volatile memory cell includes a substrate and a first insulating layer formed on the substrate. The memory cell also includes a shallow trench isolation region having walls that form an edge in the substrate and a first conducting layer having an edge and formed on the first insulating layer where the edge of the first conducting layer is aligned with the edge of the substrate formed by the walls of the shallow trench isolation region. The memory cell further includes a second insulating layer formed on the first conducting layer and a second conducting layer formed on the first insulating layer.
In another embodiment of the present invention, a method for forming a non volatile memory cell is provided. The method includes forming a tunnel oxide layer on a substrate. Forming a first conducting layer on the tunnel oxide layer having an edge. Forming a shallow trench isolation region having walls that form edges in the substrate such that the edge of the first conducting layer is aligned with one of the edges of the substrate and where the shallow trench isolation region isolates the non volatile memory cell. Forming an insulating layer on the first conducting layer and forming a second conducting layer on the insulating layer.
These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.